Multi-piece solid golf ball

ABSTRACT

The present invention provides a multi-piece solid golf ball having good shot feel, excellent flight performance and excellent controllability. The present invention relates to a multi-piece solid golf ball comprising a core composed of a center and at least one intermediate layer formed on the center, and a cover covering the core, wherein assuming that a central point hardness of the center, a surface hardness of the center, a hardness of the intermediate layer and a hardness of the cover in Shore D hardness are represented by H M , H S , H I  and H C , respectively, the H M , H S , H I  and H C  satisfy a correlation represented by the following formula: 
     H M &lt;H S &lt;H I &lt;H C   
     and the cover is formed from polyurethane material as a main component, and has a hardness in Shore D hardness of 45 to 60.

FIELD OF THE INVENTION

[0001] The present invention relates to a multi-piece solid golf ball.More particularly, it relates to a multi-piece solid golf ball havinggood shot feel, excellent flight performance and excellentcontrollability.

BACKGROUND OF THE INVENTION

[0002] In golf balls commercially selling, there are solid golf ballssuch as two-piece golf ball, three-piece golf ball and the like, andthread wound golf balls. Recently, the two-piece golf ball andthree-piece golf ball, of which flight distance can be improved whilemaintaining soft and good shot feel at the time of hitting as good asthe conventional thread wound golf ball, generally occupy the greaterpart of the golf ball market. Multi-piece solid golf balls representedby three-piece golf ball have good shot feel while maintaining excellentflight performance, because they can vary hardness distribution anddesign of golf balls, when compared with the two-piece golf ball.

[0003] The three-piece solid golf balls are obtained by inserting anintermediate layer between the core and the cover layer constituting thetwo-piece solid golf ball and have been described in Japanese PatentKokai Publication Nos. 313643/1997, 305114/1998, 151226/1998,360740/2002 and the like. In the golf balls, it has been attempted tocompromise the balance of flight performance and shot feel at the timeof hitting by using thermoplastic resin, such as polyurethane-basedthermoplastic elastomer, ionomer resin, or mixtures thereof, for theintermediate layer, to adjust a hardness, hardness distribution,deformation amount, specific gravity, elastic modulus and the like ofthe core, intermediate layer and cover to proper ranges.

[0004] In Japanese Patent Kokai Publication No. 313643/1997, athree-piece solid golf ball, of which an intermediate layer is placedbetween a core and a cover, is disclosed. The core has a center hardnessin JIS-C hardness of not more than 75 and has a surface hardness inJIS-C hardness of not more than 85, the surface hardness is higher thanthe center hardness by 5 to 25, a hardness of the intermediate layer ishigher than the surface hardness of the core by less than 10, and ahardness of the cover is higher than the hardness of the intermediatelayer. In the golf ball, since the cover is formed from ionomer resin asa base resin, the spin performance of the resulting golf ball is notsufficiently obtained, and the controllability is poor. In addition, thescuff resistance is not sufficiently obtained.

[0005] In Japanese Patent Kokai Publication No. 305114/1998, a golf ballcomprising a solid core, an intermediate layer and a cover, of which thesurface of the cover has many dimples, is disclosed. The core has asurface hardness in Shore D hardness of not more than 48, theintermediate layer has a hardness in Shore D hardness of 53 to 60 andthe hardness of the intermediate layer is higher than the surfacehardness of the core by not less than 8, the cover has a hardness inShore D hardness of 55 to 65 and the hardness of the cover is higherthan that of the intermediate layer, the dimples are consisted of twotypes having different diameter and/or depth from each other, the totalnumber of the dimples is within the range of 370 to 450, the dimplescover at least 63% of the ball surface, and the index D_(st) of theoverall dimple surface area is at least 4. In the golf ball, since thecover is formed from ionomer resin as a base resin, the hardness of thecover is high, and the shot feel and controllability are poor.

[0006] In Japanese Patent Publication No. 151226/1998, a multi-piecegolf ball, of which the center has a distortion of at least 2.5 mm undera load of 100 kg, the hardness in Shore D hardness of the intermediatelayer is at least 13 degrees higher than that of the cover, and the ballas a whole has an inertia moment of at least 83 g-cm², is described.However, since the cover has low hardness and large thickness, therebound characteristics of the resulting golf ball are poor and the spinamount is large, and the flight distance when hit by a driver is notsufficiently obtained.

[0007] In Japanese Patent Publication No. 360740/2002, a three-piecesolid golf ball comprising a core composed of a center and anintermediate layer formed on the center, and a cover formed on theintermediate layer, of which the cover is formed from a mixture ofpolyurethane-based thermoplastic elastomer and polyamide-basedthermoplastic elastomer as a base resin, is disclosed. Sine the coverhas low hardness, the spin amount of the resulting golf ball is large,and the flight distance when hit by a driver is not sufficientlyobtained.

[0008] In the conventional golf balls, sufficient performances have notbeen obtained in view of the balance of the flight performance and shotfeel, and durability at a level of practical use, as described above.Therefore, a golf ball, of which the shot feel, flight performance, anddurability are further improved, has been required.

OBJECTS OF THE INVENTION

[0009] A main object of the present invention is to provide amulti-piece solid golf ball having good shot feel, excellent flightperformance and excellent controllability.

[0010] According to the present invention, the object described abovehas been accomplished by providing a multi-piece solid golf ball, ofwhich an intermediate layer is placed between a center and a cover, byforming the cover from polyurethane material, and by adjusting ahardness distribution of the central point and surface of the center,the intermediate layer and the cover, and a hardness of the cover tospecified ranges, thereby providing a multi-piece solid golf ball havinggood shot feel, excellent flight performance and excellentcontrollability.

[0011] This object as well as other objects and advantages of thepresent invention will become apparent to those skilled in the art fromthe following description with reference to the accompanying drawings.

BRIEF EXPLANATION OF DRAWINGS

[0012] The present invention will become more fully understood from thedetailed description given hereinbelow and the accomplishing drawingswhich are given by way of illustrating only, and thus are not limitativeof the present invention, and wherein:

[0013]FIG. 1 is a schematic cross section illustrating one embodiment ofthe golf ball of the present invention.

SUMMARY OF THE INVENTION

[0014] The present invention provides a multi-piece solid golf ballcomprising a center, at least one intermediate layer formed on thecenter and a cover covering the intermediate layer, wherein

[0015] assuming that a central point hardness of the center, a surfacehardness of the center, a hardness of the intermediate layer and ahardness of the cover in Shore D hardness are represented by H_(M),H_(S), H_(I) and H_(C), respectively, the H_(M), H_(S), H_(I) and H_(C)satisfy a correlation represented by the following formula:

H_(M)<H_(S)<H_(I)<H_(C)

[0016] and

[0017] the cover is formed from polyurethane material as a maincomponent, and has a hardness in Shore D hardness of 45 to 60.

[0018] Generally, urethane cover has been widely used for golf balls,because the urethane material imparts the resulting golf ball toexcellent controllability. Particularly, soft urethane cover has beenwidely used in order to improve the controllability. However, since thesoft urethane cover had poor rebound characteristics, it was required touse an intermediate layer having high stiffness (Japanese PatentPublication No. 151226/1998). Therefore, it was problem that theresulting golf ball had also large spin amount when hit by a driver, andhit golf ball created blown-up trajectory, which reduced the flightdistance.

[0019] Moreover, golf balls satisfying the above correlation representedby the following formula:

(Central point hardness of the core)<(Surface hardness of thecore)<(Intermediate layer hardness)<(Cover hardness)

[0020] have been proposed (Japanese Patent Kokai Publication Nos.313643/1997 and 305114/1998), but it was required to use a covercomprising ionomer resin as a main component in order to have highstiffness cover. Therefore, it was problem that the cover hardness wastoo high or the spin amount at approach shot of the resulting golf ballwas small, and the controllability was not sufficiently obtained.

[0021] In the conventional golf balls, since the soft urethane cover hasbeen used as an urethane cover, there has no golf ball using urethanecover and satisfying the correlation represented by the above formula.In the golf ball of the present invention, it was accomplished todecrease the spin amount when hit by a driver, which improved the flightdistance, by forming the cover from polyurethane material as a maincomponent; and to improve the controllability at approach shot bysatisfying the above correlation represented by the following formula:

(Central point hardness of the core)<(Surface hardness of thecore)<(Intermediate layer hardness)<(Cover hardness)

[0022] Moreover, in the present invention, it was accomplished to impartthe golf ball to excellent scuff resistance by using a urethane coverhaving higher hardness than the conventional urethane cover. Therefore,even if the cover is thin, the durability of the resulting golf ball canbe sufficiently obtained.

[0023] In order to put the present invention into a more suitablepractical application, it is preferable that

[0024] the golf ball have a deformation amount of 3.0 to 4.0 mm, whenapplying from an initial load of 98 N to a final load of 1274 N; and

[0025] the polyurethane material comprise polyurethane-basedthermoplastic elastomer as a main component.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The multi-piece solid golf ball of the present invention will beexplained with reference to the accompanying drawing in detail. FIG. 1is a schematic cross section illustrating one embodiment of themulti-piece solid golf ball of the present invention. As shown in FIG.1, the golf ball of the present invention comprises a core 4 composed ofa center 1 and at least one intermediate layer 2 formed on the center,and a cover 3 covering the core. The intermediate layer 2 may havesingle-layer structure or multi-layer structure, which has two or morelayers. In FIG. 1, in order to explain the golf ball of the presentinvention simply, a golf ball having one layer of intermediate layer 2,that is, a three-piece solid golf ball will be used hereinafter forexplanation. The center 1 of the golf ball of the present invention isobtained by press-molding a rubber composition under applied heat usinga method and condition, which has been conventionally used for preparingcores of solid golf balls. The rubber composition comprises a baserubber, a co-crosslinking agent, an organic peroxide, a filler and thelike.

[0027] The base rubber used in the present invention may be also thesame one that has been conventionally used for solid golf balls, butpreferred is polybutadiene rubber, particularly so-called high-cispolybutadiene rubber containing a cis-1, 4 bond of not less than 40%,preferably not less than 80%. The high-cis polybutadiene rubber may beoptionally mixed with natural rubber, polyisoprene rubber,styrene-butadiene rubber, ethylene-propylene-diene rubber (EPDM) and thelike.

[0028] Examples of the co-crosslinking agents are not limited, butinclude α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms(such as acrylic acid, methacrylic acid, etc.) or mono or divalent metalsalts (such as zinc or magnesium salts) thereof, or a combinationthereof. Preferred is zinc acrylate or zinc methacrylate, because theyimpart high rebound characteristics to the resulting golf ball. Theamount of the co-crosslinking agent is preferably from 10 to 40 parts byweight, more preferably from 10 to 35 parts, most preferably from 15 to30 parts by weight, based on 100 parts by weight of the base rubber.When the amount of the co-crosslinking agent is smaller than 10 parts byweight, the center is not sufficiently crosslinked, and the reboundcharacteristics and the durability are degraded. On the other hand, whenthe amount of the co-crosslinking agent is larger than 40 parts byweight, the center is too hard, and the shot feel of the resulting golfball is hard and poor.

[0029] Examples of the organic peroxides include dicumyl peroxide,1,1-bis (t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane, di-t-butyl peroxide and thelike. The preferred organic peroxide is dicumyl peroxide. The amount ofthe organic peroxide is preferably from 0.1 to 3.0 parts by weight, morepreferably from 0.3 to 2.5 parts by weight, most preferably from 0.5 to2.0 parts by weight based on 100 parts by weight of the base rubber.When the amount of the organic peroxide is smaller than 0.1 parts byweight, the center is not sufficiently vulcanized. On the other hand,when the amount of the organic peroxide is larger than 3.0 parts byweight, the center is hard, but the rebound characteristics are notsufficiently improved. In addition, the shot feel is poor.

[0030] The filler, which can be typically used for the core of solidgolf ball, includes for example, inorganic filler (such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide and the like), highspecific gravity metal powder filler (such as tungsten powder,molybdenum powder and the like), and the mixture thereof. The amount ofthe filler is preferably from 1 to 30 parts by weight, more preferablyfrom 3 to 20 parts by weight, based on 100 parts by weight of the baserubber. When the amount of the filler is smaller than 1 part by weight,it is difficult to adjust the weight of the resulting golf ball. On theother hand, when the amount of the filler is larger than 30 parts byweight, the weight ratio of the rubber component in the center is small,and the rebound characteristics of the resulting golf ball are degradedtoo much.

[0031] Where appropriate, it is possible to compound a component whichis typically used in the manufacture of solid golf ball cores togetherwith the rubber composition; e.g., other additives such as organicsulfides, antioxidants and the like. If used, preferably the amount ofthe additives is preferably 0.5 to 5.0 parts by weight, more preferably0.7 to 4.0 parts by weight, based on 100 parts by weight of the baserubber.

[0032] The center 1 used for the golf ball of the present invention canbe obtained by mixing, and then vulcanizing and press-molding the aboverubber composition under applied heat in a mold. The vulcanizationcondition is not limited, but the vulcanization may be conducted at 140to 180° C. and 2.8 to 11.8 MPa for 10 to 60 minutes.

[0033] In the golf ball of the present invention, it is suitable for thecenter 1 to have a diameter of 25 to 41 mm, preferably 27 to 40 mm. Whenthe diameter of the center 1 is smaller than 25 mm, it is required toincrease the thickness of the intermediate layer or the cover, and therebound characteristics of the resulting golf ball are degraded. On theother hand, when the diameter of the center is larger than 41 mm, thethickness of the intermediate layer is too small, and it is difficult tomold the intermediate layer.

[0034] In the golf ball of the present invention, it is desired for thecenter 1 to have a deformation amount when applying from an initial loadof 98 N to a final load of 1274 N of 3.5 to 5.5 mm, preferably 3.5 to5.0 mm, more preferably 3.8 to 4.8 mm, most preferably 4.0 to 4.5 mm.When the deformation amount of the center is smaller than 3.5 mm, thecenter is too hard, and the shot feel of the resulting golf ball is hardand poor. On the other hand, when the deformation amount is larger than5.5 mm, the center is too soft, and the shot feel of the resulting golfball is heavy and poor.

[0035] In the present invention, assuming that a central point hardnessof the center, a surface hardness of the center, a hardness of theintermediate layer and a hardness of the cover in Shore D hardness arerepresented by HM, Hs, H_(I) and H_(C), respectively, it is required forthe golf ball to satisfy a correlation represented by the followingformula: H_(M)<H_(S)<H_(I)<H_(C) Therefore, in the center 1 of the golfball of the present invention, it is required that the surface hardness(H_(S)) is higher than the central point hardness (H_(M)), and thehardness difference thereof (H_(S)−H_(M)) is preferably 5 to 30, morepreferably 8 to 30, most preferably 10 to 25. When the surface hardnessis not more than the central point hardness, the hardness distributionsuch that the outer portion is hard and the inner portion is soft in thewhole golf ball are not obtained, and the spin amount is large, whichreduces the flight distance. In addition, when the hardness differenceis smaller than 5, the technical effects accomplished by having hardnessdistribution such that the outer portion is hard and the inner portionis soft in the whole golf ball are not sufficiently obtained, and thespin amount is large, which reduces the flight distance. On the otherhand, when the hardness difference is larger than 30, the durability ispoor.

[0036] In the golf ball of the present invention, it is desired for thecenter 1 to have a central point hardness (H_(M)) in Shore D hardness of20 to 40, preferably 22 to 38, more preferably 25 to 35. When thecentral point hardness is lower than 20, the center is too soft, and therebound characteristics of the resulting golf ball are degraded, whichreduces the flight distance. On the other hand, the central pointhardness is higher than 40, it is difficult to satisfy the correlationrepresented by the above formula, and the hardness distribution suchthat the outer portion is hard and the inner portion is soft in thewhole golf ball are not obtained. Therefore, the spin amount is large,which reduces the flight distance. The term “a central point hardness ofthe center” as used herein refers to the hardness, which is determinedby cutting the resulting center into two equal parts and then measuringa hardness at its central point in section.

[0037] In the golf ball of the present invention, it is desired for thecenter 1 to have the surface hardness (H_(S)) in Shore D hardness of 35to 55, preferably 38 to 52, more preferably 40 to 50. When the surfacehardness is lower than 35, the center is too soft, and the reboundcharacteristics of the resulting golf ball are degraded, which reducesthe flight distance. On the other hand, when the surface hardness ishigher than 55, the intermediate layer and cover are too hard in orderto satisfy the correlation represented by the above formula, and theshot feel of the resulting golf ball is poor. The term “a surfacehardness of the center” as used herein refers to the hardness, which isdetermined by measuring a hardness at the surface of the resultingcenter. The intermediate layer 2 is then formed on the center 1.

[0038] The intermediate layer 2 of the golf ball of the presentinvention may be formed from the rubber composition as used for thecenter 1, or from thermoplastic resin as a base resin. Examples of thethermoplastic resins include one or combination of two or more selectedfrom the group consisting of ionomer resin and thermoplastic elastomer,which can be typically used for the cover of golf balls. Examples of thethermoplastic elastomers include polyester-based thermoplasticelastomer, polyamide-based thermoplastic elastomer, polyurethane-basedthermoplastic elastomer, polyolefin-based thermoplastic elastomer,styrene-based thermoplastic elastomer and the like. The thermoplasticelastomers may have functional groups, such as carboxyl group, glycydylgroup, sulfone group, epoxy group and the like.

[0039] When using the rubber composition for the intermediate layer 2,the amount of the co-crosslinking agent and organic peroxide is slightlydifferent from the rubber composition used for the center 1 in order tosatisfy the correlation represented by the above formula. That is, theamount of the co-crosslinking agent is preferably from 15 to 50 parts byweight, more preferably from 20 to 45 parts by weight, based on 100parts by weight of the base rubber. Preferred are zinc acrylate, zincmethacrylate, magnesium acrylate or magnesium methacrylate. The amountof the organic peroxide is preferably from 0.1 to 6.0 parts by weight,more preferably from 0.3 to 5.0 parts by weight, most preferably from0.5 to 4.0 parts by weight based on 100 parts by weight of the baserubber.

[0040] The ionomer resin may be a copolymer of α-olefin andα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, of which aportion of carboxylic acid groups is neutralized with metal ion, aterpolymer of α-olefin, α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and α,β-unsaturated carboxylic acid ester, of which aportion of carboxylic acid groups is neutralized with metal ion ormixture thereof. Examples of the α-olefins in the ionomer preferablyinclude ethylene, propylene and the like. Examples of theα,β-unsaturated carboxylic acid in the ionomer include acrylic acid,methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like,preferred are acrylic acid and methacrylic acid. Examples of theα,β-unsaturated carboxylic acid ester in the ionomer include methylester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester ofacrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acidand the like. Preferred are acrylic acid esters and methacrylic acidesters. The metal ion, which neutralizes a portion of carboxylic acidgroups of the copolymer or terpolymer, includes an alkali metal ion,such as a sodium ion, a potassium ion, a lithium ion and the like; adivalent metal ion, such as a zinc ion, a calcium ion, a magnesium ionand the like; a trivalent metal ion, such as an aluminum, a neodymiumion and the like; and mixture thereof. Preferred are sodium ions, zincions, lithium ions and the like, in view of rebound characteristics,durability and the like.

[0041] The ionomer resin is not limited, but examples thereof will beshown by a trade name thereof. Examples of the ionomer resins, which arecommercially available from Du Pont-Mitsui Polychemicals Co., Ltd.include Hi-milan 1555, Hi-milan 1557, Hi-milan 1601, Hi-milan 1605,Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan1707, Hi-milan 1855, Hi-milan 1856, Hi-milan AM7316 and the like.Examples of the ionomer resins, which are commercially available from DuPont Co., include Surlyn 8945, Surlyn 9945, Surlyn 6320, Surlyn 8320,Surlyn AD8511, Surlyn AD8512 and the like. Examples of the ionomerresins, which are commercially available from Exxon Chemical Co.,include Iotek 7010, Iotek 8000 and the like. These ionomer resins may beused alone or in combination.

[0042] Examples of the thermoplastic elastomers, which are commerciallyavailable, include polyester-based thermoplastic elastomer, which iscommercially available from Toray-Do Pont Co., Ltd. under the trade nameof “Hytrel” (such as “Hytrel 3548”, “Hytrel 4047”); polyamide-basedthermoplastic elastomer, which is commercially available from AtofinaJapan Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”);polyurethane-based thermoplastic elastomer, which is commerciallyavailable from BASF Japan Co., Ltd. under the trade name of “Elastollan”(such as “Elastollan ET880”); olefin-based thermoplastic elastomeravailable from Mitsubishi Chemical Co., Ltd. under the trade name“Thermoran” (such as “Thermoran 3981N”); polyolefin-based thermoplasticelastomer, which is commercially available from Sumitomo Chemical Co.,Ltd. under the trade name of “Sumitomo TPE” (such as “Sumitomo TPE3682”and “Sumitomo TPE9455”); styrene-based thermoplastic elastomer, whichare commercially available from Mitsubishi Chemical Co., Ltd. under thetrade name of “Rabalon” (such as “Rabalon SR04”); styrene-basedthermoplastic elastomer available from Asahi Kasei corporation under thetrade name “Tuftec” (such as “Tuftec H1051”); and the like.

[0043] The composition for the intermediate layer 2 used in the presentinvention may optionally contain fillers, pigments and the otheradditives such as an antioxidant in addition to the thermoplastic resinas a base resin.

[0044] Examples of the fillers include inorganic filler (such as zincoxide, barium sulfate, calcium carbonate and the like), high specificgravity metal powder filler (such as tungsten powder, molybdenum powderand the like), and the mixture thereof.

[0045] The intermediate layer 2 of the present invention may be formedby conventional methods, which have been known in the art and used forforming the cover of the golf balls. When the intermediate layer 2 isformed from the rubber composition, the rubber composition for theintermediate layer is mixed, and coated on the center 1 into aconcentric sphere, and then vulcanized by press-molding at 160 to 180°C. for 10 to 20 minutes in the mold to obtain a core 4, which is formedby covering the intermediate layer 2 on the center 1. When theintermediate layer 2 is formed from thermoplastic resin, the resincomposition for the intermediate layer is molded into a semi-sphericalhalf-shell, and the center is covered with the two half-shells, followedby pressure molding; or the resin composition for the intermediate layeris injection molded directly on the center 1; to obtain the core 4. Itis preferable for the surface of the resulting core to be buffed toimprove the adhesion to the cover formed on the core.

[0046] In the present invention, assuming that a central point hardnessof the center, a surface hardness of the center, a hardness of theintermediate layer and a hardness of the cover in Shore D hardness arerepresented by H_(M), H_(S), H_(I) and H_(C), respectively, it isrequired for the golf ball to satisfy a correlation represented by thefollowing formula: H_(M)<H_(S)<H_(I)<H_(C) as described above.Therefore, in the golf ball of the present invention, it is requiredthat the hardness (H_(I)) of the intermediate layer 2 is higher than thesurface hardness (H_(S)) of the center 1, and the hardness differencethereof (H_(I)−H_(S)) is preferably 1 to 15, more preferably 2 to 10,most preferably 3 to 8. When the hardness (H_(I)) of the intermediatelayer 2 is not more than the surface hardness (H_(S)) of the center 1,the hardness distribution such that the outer portion is hard and theinner portion is soft in the whole golf ball are not obtained, and thespin amount is large, which reduces the flight distance. In addition,when the hardness difference is smaller than 1, the technical effectsaccomplished by having hardness distribution such that the outer portionis hard and the inner portion is soft in the whole golf ball are notsufficiently obtained, and the spin amount is large, which reduces theflight distance. On the other hand, when the hardness difference islarger than 15, the intermediate layer is too hard, and thecontrollability and shot feel of the resulting golf ball are degraded.

[0047] In the golf ball of the present invention, it is desired for theintermediate layer 2 to have a hardness (H_(I)) in Shore D hardness of38 to 58, preferably 40 to 55, more preferably 42 to 52. When thehardness of the intermediate layer is lower than 38, the core is toosoft, and the rebound characteristics and durability of the resultinggolf ball are degraded. On the other hand, the hardness of theintermediate layer is higher than 58, the shot feel of the resultinggolf ball is poor. In addition, the cover is too hard in order tosatisfy the correlation represented by the above formula, and thecontrollability at approach shot of the resulting golf ball is degraded.

[0048] It is desired for the intermediate layer 2 to have a thickness,which is determined by the diameter of the center 1 and core 4, of 0.5to 4.0 mm, preferably 0.5 to 3.0 mm, more preferably 0.7 to 2.0 mm. Whenthe thickness of the intermediate layer is smaller than 0.5 mm, thetechnical effects accomplished by the presence of the intermediate layerare not sufficiently obtained, and the rebound characteristics aredegraded and the spin amount is increased, which reduces the flightdistance. On the other hand, when the thickness is larger than 4.0 mm,the shot feel of the resulting golf ball is hard and poor, because theintermediate layer is relatively hard. The cover 3 is then formed on theintermediate layer 2.

[0049] In the golf ball of the present invention, it is suitable for thecore 4 to have a diameter of 39.0 to 42.2 mm, preferably 40.0 to 42.0mm, more preferably 40.5 to 42.0 mm. When the diameter of the core issmaller than 39.0 mm, the cover is thick, and the reboundcharacteristics of the resulting golf ball are degraded. On the otherhand, when the diameter of the core is larger than 42.2 mm, the cover istoo thin, and the technical effects accomplished by the presence of thecover are not sufficiently obtained. In addition, it is difficult tomold the cover.

[0050] In the golf ball of the present invention, it is desired for thecore 4 to have a deformation amount when applying from an initial loadof 98 N to a final load of 1274 N of 3.2 to 5.0 mm, preferably 3.2 to4.8 mm, more preferably 3.4 to 4.5 mm, most preferably 3.2 to 3.8 mm.When the deformation amount of the core is smaller than 3.2 mm, the shotfeel of the resulting golf ball is hard and poor. On the other hand,when the deformation amount is larger than 5.0 mm, the core is too soft,and the shot feel of the resulting golf ball is heavy and poor.

[0051] The cover 3 is then covered on the core 4. In the golf ball ofthe present invention, it is required for the cover 3 to be formed frompolyurethane material as a main component. In the polyurethanematerials, there are thermosetting type and thermoplastic typepolyurethane materials, but preferred are thermoplastic typepolyurethane materials, such as polyurethane-based thermoplasticelastomer, in view of processability and cost.

[0052] Polyurethane-based thermoplastic elastomer generally containspolyurethane structure as hard segment and polyester or polyether assoft segment. The polyurethane structure generally contains diisocyanateand curing agent, such as amine-based curing agent. Thepolyurethane-based thermoplastic elastomer includes polyurethane-basedthermoplastic elastomer that the diisocyanate is aromatic diisocyanate,cycloaliphatic diisocyanate or aliphatic diisocyanate.

[0053] Examples of the aromatic diisocyanate include tolylenediisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), tolidine diisocyanate (TODI),xylylene diisocyanate (XDI) and the like. Preferred is MDI. Concreteexamples of the polyurethane-based thermoplastic elastomer formed byusing the MDI include polyurethane-based thermoplastic elastomer, whichis commercially available from BASF Japan Co., Ltd. under the trade nameof “Elastollan ET890”, and the like.

[0054] Examples of the cycloaliphatic diisocyanates include4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), which is hydrogenatedcompound of MDI; 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), which ishydrogenated compound of XDI; isophorone diisocyanate (IPDI); andtrans-1,4-cyclohexane diisocyanate (CHDI). Preferred is the H₁₂MDI inview of general-purpose properties and processability. Concrete examplesof the polyurethane-based thermoplastic elastomer formed by using theH₁₂MDI include polyurethane-based thermoplastic elastomers, which arecommercially available from BASF Japan Co., Ltd. under the trade name of“Elastollan XNY90A”, “Elastollan XNY97A”, “Elastollan XNY585”,“Elastollan XKP016”, and the like.

[0055] Examples of the aliphatic diisocyanates include hexamethylenediisocyanate (HDI), lysine diisocyanate (LDI), and the like. Preferredis HDI. Concrete examples of the polyurethane-based thermoplasticelastomer formed by using the HDI include polyurethane-basedthermoplastic elastomer, which is commercially available from DainipponInk & Chemicals Inc. under the trade name of “Pandex T-7890”(tradename), and the like.

[0056] Preferred are polyurethane-based thermoplastic elastomers formedby using diisocyanate having no double bond in backbone structure inmolecule, that is, aliphatic diisocyanate and cycloaliphaticdiisocyanate in view of yellowing resistance. Preferred arepolyurethane-based thermoplastic elastomers formed by usingcycloaliphatic diisocyanate and aromatic diisocyanate, which have highmechanical strength, in view of scuff resistance. Therefore, in thepresent invention, preferred is polyurethane-based thermoplasticelastomer formed by using cycloaliphatic diisocyanate in view of boththe yellowing resistance and scuff resistance.

[0057] For the cover 3 of the golf ball of the present invention, theabove polyurethane-based thermoplastic elastomer may be used alone, orthe polyurethane-based thermoplastic elastomer may be used incombination with at least one of the ionomer resin and the thermoplasticelastomer as used for the intermediate layer 2. When using thecombination, polyamide-based thermoplastic elastomer is most preferablein view of the compatibility with the polyurethane-based thermoplasticelastomer and the rebound characteristics, and a weight ratio (a/b) ofthe polyurethane-based thermoplastic elastomer (a) to thepolyamide-based thermoplastic elastomer (b) is preferably 95/5 to 70/30.

[0058] In the golf ball of the present invention, the cover compositionmay optionally contain fillers such as barium sulfate, pigments such astitanium dioxide, and other additives (such as a dispersant, anantioxidant, a UV absorber, a photostabilizer and a fluorescent agent ora fluorescent brightener, etc.), in addition to the base resin as a maincomponent, as long as the addition of the additive does not deterioratethe desired performance of the golf ball cover. If used, the amount ofthe pigment is preferably 0.1 to 5.0 parts by weight, based on the 100parts by weight of the base resin of the cover.

[0059] In the golf ball of the present invention, it is desired for thecover 3 to have a thickness of 0.3 to 2.0 mm, preferably 0.3 to 1.5 mm,more preferably 0.5 to 1.0 mm. When the thickness of the cover issmaller than 0.3 mm, the technical effects accomplished by the presenceof the cover are not sufficiently obtained, and the controllability anddurability are degraded. On the other hand, when the thickness is largerthan 2.0 mm, the technical effects accomplished by the presence of thecore and intermediate layer are not sufficiently obtained, and therebound characteristics of the resulting golf ball are degraded, whichreduces the flight distance.

[0060] In the present invention, assuming that a central point hardnessof the center, a surface hardness of the center, a hardness of theintermediate layer and a hardness of the cover in Shore D hardness arerepresented by H_(M), H_(S), H_(I) and H_(C), respectively, it isrequired for the golf ball to satisfy a correlation represented by thefollowing formula: H_(M)<H_(S)<H_(I)<H_(C) as described above.Therefore, in the golf ball of the present invention, it is requiredthat the hardness (H_(C)) of the cover is higher than the hardness(H_(I)) of the intermediate layer 2, and the hardness difference thereof(H_(C)−H_(I)) is preferably 1 to 15, more preferably 3 to 12, mostpreferably 5 to 10. When the hardness (H_(C)) of the cover 3 is not morethan the hardness (H_(I)) of the intermediate layer 2, the hardnessdistribution such that the outer portion is hard and the inner portionis soft in the whole golf ball are not obtained, and the spin amount islarge, which reduces the flight distance. In addition, when the hardnessdifference is smaller than 1, the technical effects accomplished byhaving hardness distribution such that the outer portion is hard and theinner portion is soft in the whole golf ball are not sufficientlyobtained, and the spin amount is large, which reduces the flightdistance. On the other hand, when the hardness difference is larger than15, the cover is too hard, and the controllability and shot feel of theresulting golf ball are degraded.

[0061] In the golf ball of the present invention, it is desired for thecover 3 to have a hardness (H_(C)) in Shore D hardness of 45 to 60,preferably 47 to 60, more preferably 50 to 58. When the hardness of thecover is lower than 45, the deformation amount of the surface of theresulting golf ball at the time of hitting is large even if adjustingthe hardness of the core, and the rebound characteristics are degraded,which reduces the flight distance. In addition, the durability is poor.The term “a hardness of the intermediate layer” and “a hardness of thecover” as used herein refer to the hardness, which is determined bymeasuring a hardness using a sample of a stack of the three or more heatand press molded sheets having a thickness of about 2 mm from theresulting compositions for the intermediate layer and cover, which hadbeen stored at 23° C. for 2 weeks.

[0062] The cover of the present invention may be formed by the samemethods as used in the intermediate layer. At the time of molding thecover, many depressions called “dimples” are formed on the surface ofthe golf ball. The term “an area of the dimple” as used herein refers tothe area enclosed in the periphery (edge) of the dimple when observingthe central point of the golf ball from infinity, which is the area ofplane. When the dimple is spherical, the area of the dimple S isdetermined by calculating from the following formula:

S=π(d/2)²

[0063] wherein “d” is a diameter of the dimple. The ratio of the golfball surface occupied by the dimple to the total surface area of thegolf ball is determined by calculating a ratio of (the total of the area“S” of each dimple) to (the-surface area of the phantom-sphere assumingthat the golf ball is a true sphere having no dimples on the surfacethereof).

[0064] In the golf ball of the present invention, it is desired for thedimple to have a ratio of the golf ball surface occupied by the dimpleof 70 to 90%, preferably 72 to 88%, more preferably 74 to 86%, based onthe total surface area of the golf ball. When the ratio of the golf ballsurface occupied by the dimple is smaller than 70%, the technicaleffects of improving the flight distance accomplished by the presence ofthe dimple are not sufficiently obtained, which reduces the flightdistance. On the other hand, when the ratio of the golf ball surfaceoccupied by the dimple is larger than 90%, the depressions on thesurface of the golf ball are too many, and the shape of the golf ball isnot spherical to break symmetry of airflow around the golf ball on thefly. In addition, since there is not sufficient space between dimplesadjacent to each other on the surface of the golf ball, it is difficultto design the dimples having the ratio of the golf ball surface occupiedby the dimples of larger than 90%. The total volume of the dimples andthe ratio of the golf ball surface occupied by the dimples as usedherein are determined by measuring at the surface of the resulting golfball, and if paint is applied on the cover, they are determined bymeasuring at the surface of the applied golf ball.

[0065] In the golf ball of the present invention, it is desired for thedimple to have total number of 250 to 500, preferably 300 to 480, morepreferably 320 to 450. When the total number of the dimples is smallerthan 250, the technical effects accomplished by the presence of thedimples are not sufficiently obtained. On the other hand, when the totalnumber of the dimples is larger than 500, the size of each dimple issmall, and the technical effects accomplished by the presence of thedimples are not sufficiently obtained. Therefore, in the both cases, thetechnical effects accomplished by the presence of the dimples are notsufficiently obtained to improve the flight performance.

[0066] Furthermore, paint finishing or marking with a stamp may beoptionally provided after the cover is molded for commercial purposes.

[0067] In the golf ball of the present invention, it is desired to havea deformation amount when applying from an initial load of 98 N to afinal load of 1274 N of 3.0 to 4.0 mm, preferably 3.0 to 3.8 mm, morepreferably 3.2 to 3.8 mm. When the deformation amount is smaller than3.0 mm, the golf ball is too hard, and the shot feel is hard and poor.On the other hand, when the deformation amount is larger than 4.0 mm,the golf ball is too soft, and the rebound characteristics are degraded,which reduces the flight distance.

[0068] The multi-piece solid golf ball of the present invention isformed to a diameter of at least 42.67 mm (preferably 42.67 to 42.82 mm)and a weight of no more than 45.93 g, in accordance with the regulationsfor golf balls.

[0069] The diameter of golf balls is limited to not less than 42.67 mmin accordance with the regulations for golf balls as described above.Generally, when the diameter of the golf ball is large, air resistanceof the golf ball on the fly is large, which reduces the flight distance.Therefore, most of golf balls commercially available are designed tohave a diameter of 42.67 to 42.82 mm. The present invention isapplicable to the golf balls having the diameter. There are golf ballshaving large diameter in order to improve the easiness of hitting. Inaddition, there are cases where golf balls having a diameter out of theregulations for golf balls are required depending on the demand andobject of users. Therefore, it can be considered for golf balls to havea diameter of 42 to 44 mm, more widely 40 to 45 mm. The presentinvention is also applicable to the golf balls having the diameter.

EXAMPLES

[0070] The following Examples and Comparative Examples furtherillustrate the present invention in detail but are not to be construedto limit the scope of the present invention.

[0071] Production of Core

[0072] (i) Production of Center

[0073] The rubber compositions for the center having the formulationsshown in Table 1 were mixed, and then vulcanized by press-molding at160° C. for 20-minutes in a mold to obtain spherical centers. Thediameter, deformation amount, the central point hardness and the surfacehardness of the resulting center were measured. The results are shown inTable 4 (Examples) and Table 5 (Comparative Examples). Test methods aredescribed later. TABLE 1 (parts by weight) Center composition I II IIIBR-11 *1 100 100 100 Zinc acrylate 26 24 20 Zinc oxide 5 5 5 Dicumylperoxide 0.7 0.7 0.7 Diphenyl disulfide 0.5 0.5 0.5 Barium sulfate *2proper proper proper amount amount amount

[0074] (ii) Preparation of Intermediate Layer Composition

[0075] The formulation materials shown in Table 2 were mixed using akneading type twin-screw extruder to obtain pelletized intermediatelayer compositions. The extrusion condition was,

[0076] a screw diameter of 45 mm,

[0077] a screw speed of 200 rpm, and

[0078] a screw L/D of 35.

[0079] The formulation materials were heated at 200 to 260° C. at thedie position of the extruder. The hardness of the intermediate layer wasmeasured, using a sample of a stack of the three or more heat and pressmolded sheets having a thickness of about 2 mm from the resultingcomposition for the intermediate layer, which had been stored at 23° C.for 2 weeks. The results are shown in Table 2, Table 4 (Examples) andTable 5 (Comparative Examples). TABLE 2 (parts by weight) Intermediatelayer composition A B C D Hi-milan 1555 *3 — — — 25 Hi-milan 1557 *4 — —— 25 Surlyn 8945 *5 30 25 50 — Surlyn 9945 *6 30 25 50 — Rabalon SR04 *740 50 — 50 Shore D hardness 46 43 64 40

[0080] (iii) Preparation of Two-Layered Core

[0081] The resulting intermediate layer compositions were directlyinjection molded on the center produced in the (ii) to form a sphericaltwo-layered core. The thickness of the resulting intermediate layer, thediameter and deformation amount of the resulting two-layered core weremeasured, and the results are shown in Table 4 (Examples) and Table 5(Comparative Examples).

[0082] Preparation of Cover Composition

[0083] The formulation materials shown in Table 3 were mixed using akneading type twin-screw extruder to obtain pelletized covercompositions. The extrusion condition was,

[0084] a screw diameter of 45 mm,

[0085] a screw speed of 200 rpm, and

[0086] a screw L/D of 35.

[0087] The formulation materials were heated at 200 to 260° C. at thedie position of the extruder. The hardness of the cover was measured,using a sample of a stack of the three or more heat and press moldedsheets having a thickness of about 2 mm from the resulting compositionfor the cover, 5 which had been stored at 23° C. for 2 weeks. Theresults are shown in Table 3, Table 4 (Examples) and Table 5(Comparative Examples). TABLE 3 (parts by weight) Cover composition a bc d e Elastollan XNY97A *8 30 60 — — — Elastollan XNY90A *9 — — — — 90Elastollan XKP-016 *10 60 30 — — — Pebax 5533 *11 10 10 — — 10 Surlyn8945 *5 — — 50 50 — Surlyn 9945 *6 — — 50 — — Surlyn 6320 *12 — — — 50 —Titanium dioxide  4  4  4  4  4 Shore D hardness 55 52 64 53 42

Examples 1 to 5 and Comparative Examples 1 to 5

[0088] The resulting cover compositions were directly injection moldedon the intermediate layer to form a cover layer having a thickness shownin Table 4 (Examples) and Table 5 (Comparative Examples). Then, clearpaint was coated on the surface to obtain a golf ball having a weight of45.4 g and a diameter of 42.7 mm. The ratio of the golf ball surfaceoccupied by the dimple was 84% and the total number of the dimples was410. With respect to the resulting golf balls, the deformation amount,flight performance (spin amount and flight distance) were measured, andthe shot feel, controllability and scuff resistance were evaluated. Theresults are shown in Table 6 (Examples) and Table 7 (ComparativeExamples). The test methods are as follows.

(Test Method)

[0089] (1) Deformation Amount of Center

[0090] The deformation amount of center was determined by measuring adeformation amount when applying from an initial load of 98 N to a finalload of 1274 N on the center.

[0091] (2) Hardness

[0092] (i) Hardness of center

[0093] The surface hardness of the center was determined by measuring ahardness at the surface of the resulting center. The central pointhardness was determined by cutting the resulting center into two equalparts and then measuring a hardness at its central point in section. Thehardness was measured using a Shore D hardness meter according to ASTM D2240-68.

[0094] (ii) Hardness of Intermediate Layer and Cover

[0095] The hardness of the intermediate layer and cover were determinedby measuring a hardness, using a sample of a stack of the three or moreheat and press molded sheets (slab) having a thickness of about 2 mmfrom the intermediate layer composition and cover composition, which hadbeen stored at 23° C. for 2 weeks, with a Shore D hardness meteraccording to ASTM D 2240-68.

[0096] (3) Flight Performance

[0097] (i) Flight Performance 1

[0098] After a No. 1 wood club (W#1, a driver) having metal head wasmounted to a swing robot manufactured by Golf Laboratory Co. and theresulting golf ball was hit at a head speed of 45 m/sec, the spin amount(backspin amount) immediately after hitting and flight distance weremeasured. As the flight distance, total that is a distance to the stoppoint of the hit golf ball was measured. The measurement was conducted 5times for each golf ball (n=5), and the average is shown as the resultof the golf ball.

[0099] (ii) Flight Performance 2

[0100] After a sand wedge (SW) was mounted to a swing robot manufacturedby Golf Laboratory Co. and the golf ball was hit at a head speed of 20m/sec, the launch angle, spin amount and flight distance were measured.The spin amount (backspin amount) immediately after hitting wasmeasured. The measurement was conducted 5 times for each golf ball(n=5), and the average is shown as the result of the golf ball.

[0101] (4) Shot Feel

[0102] The shot feel of the golf ball is evaluated by 10 golfersaccording to a practical hitting test using a No. 1 wood club (W#1, adriver) having a metal head. The results shown in the Tables below arebased on the fact that most of golfers evaluated with the same criterionabout shot feel. The evaluation criteria are as follows.

[0103] Evaluation criteria

[0104] ∘: The golfers felt that the golf ball has good shot feel suchthat impact force at the time of hitting is small and reboundcharacteristics are good.

[0105] Δ: The golfers felt that the golf ball has fairly good shot feel.

[0106] x: The golfers felt that the golf ball has poor shot feel suchthat impact force at the time of hitting is large or the golf ball hasheavy and poor shot feel.

[0107] (5) Controllability

[0108] The controllability of the golf ball is evaluated by high-level10 golfers according to a practical hitting test using a pitting wedge(PW). The evaluation criteria are as follows. The results shown in theTables below are based on the fact that most of golfers evaluated withthe same criterion about controllability.

[0109] Evaluation Criteria

[0110] ∘: The golfers felt that it is easy to apply spin on the golfball, and the golf ball has good controllability.

[0111] Δ: The golfers felt that the golf ball has fairly goodcontrollability.

[0112] x: The golfers felt that it is difficult to apply spin on thegolf ball such that the golf ball slips on the face of golf club, andthe golf ball has poor controllability.

[0113] (6) Scuff Resistance

[0114] After a pitching wedge (PW) commercially available was mounted toa swing robot manufactured by Golf Laboratory Co., two points on thesurface of each golf ball was hit at a head speed of 36 m/sec one timefor each point. The two points were evaluated by checking the surfaceappearance by visual observation. The evaluation criteria are asfollows.

[0115] Evaluation Criteria

[0116] ∘: The surface of the golf ball slightly has a cut, but it is notparticularly noticeable.

[0117] Δ: The surface of the golf ball clearly has a cut, and thesurface becomes fluffy.

[0118] x: The surface of the golf ball is considerably chipped off, andthe surface noticeably becomes fluffy.

Test Results

[0119] TABLE 4 Example No. Test item 1 2 3 4 5 (Center) Composition I II I II Diameter (mm) 37.7 37.7 36.7 38.5 36.7 Deformation 4.1 4.1 4.24.0 4.5 amount (mm) Central point 28 28 28 29 28 hardness (Shore D)Surface hardness 43 43 42 44 41 (Shore D) (intermediate layer)Composition A A A A B Thickness (mm) 1.3 1.3 1.6 1.3 1.6 Hardness (ShoreD) 46 46 46 46 43 (Core) Diameter (mm) 40.3 40.3 39.9 41.1 40.3Deformation 3.7 3.7 3.6 3.7 4.1 amount (mm) (Cover) Composition a b b bb Thickness (mm) 1.2 1.2 1.4 0.8 1.2 Hardness (Shore D) 55 52 52 52 52

[0120] TABLE 5 Comparative Example No. Test item 1 2 3 4 5 (Center)Composition I I I II III Diameter (mm) 37.7 37.7 37.7 36.7 37.7Deformation 4.1 4.1 4.1 4.1 5.5 amount (mm) Central point 29 29 29 29 26hardness (Shore D) Surface hardness 43 43 43 43 38 (Shore D)(intermediate layer) Composition C A A D D Thickness (mm) 1.3 1.3 1.31.3 1.3 Hardness (Shore D) 64 46 46 40 40 (Core) Diameter (mm) 40.3 40.340.3 40.3 40.3 Deformation 3.3 3.7 3.7 3.8 4.7 amount (mm) (Cover)Composition a c d b e Thickness (mm) 1.2 1.2 1.2 1.2 1.2 Hardness (ShoreD) 55 64 53 52 42

[0121] TABLE 6 Example No. Test item 1 2 3 4 5 (Golf ball) Deformation3.3 3.4 3.2 3.5 3.8 amount (mm) Flight performance 1 (W#1; 45 m/sec)Spin amount (rpm) 2630 2720 2760 2680 2550 Total (m) 223.5 221.61 222.0222.7 221.2 Flight performance 2 (SW; 20 m/sec) Spin amount (rpm) 64606620 6710 6600 6530 Shot feel ◯ ◯ ◯ ◯ Δ Controllability ◯ ◯ ◯ ◯ ◯ Scuffresistance ◯ ◯ ◯ ◯ ◯

[0122] TABLE 7 Comparative Example No. Test item 1 2 3 4 5 (Golf ball)Deformation 3.0 2.9 3.3 3.4 4.2 amount (mm) Flight performance 1 (W#1;45 m/sec) Spin amount (rpm) 2820 2650 2750 2900 2850 Total (m) 220.3224.5 221.9 217.0 211.3 Flight performance 2 (SW; 20 m/sec) Spin amount(rpm) 6490 4830 6200 6770 6830 Shot feel X X ◯ Δ X Controllability ◯ X Δ◯ ◯ Scuff resistance ◯ ◯ X ◯ ◯

[0123] As is apparent from Tables 6 and 7, the golf balls of Examples 1to 5 of the present invention, when compared with the golf balls ofComparative Examples 1 to 5, had good shot feel, excellent flightperformance and excellent controllability.

[0124] On the other hand, in the golf ball of Comparative Example 1,since the hardness of the intermediate layer is very high, the hardnessdistribution such that the outer portion is hard and the inner portionis soft in the whole golf ball are not obtained, and the spin amount islarge, which reduces the flight distance. In addition, the impact forceat the time of hitting is large, and the shot feel is poor. In the golfball of Comparative Example 2, since the cover is formed from ionomerresin, the cover hardness is high, and the spin amount is small, whichdegrades the controllability. In addition, the impact force is large,and the shot feel is very poor.

[0125] In the golf ball of Comparative Example 3, since the cover isformed from the mixture of hard ionomer resin and soft ionomer resin,the shot feel is good, but the scuff resistance is very poor. In thegolf ball of Comparative Example 4, since the hardness of theintermediate layer is low, the hardness distribution such that the outerportion is hard and the inner portion is soft in the whole golf ball arenot obtained, and the spin amount is large, which reduces the flightdistance. In addition, the shot feel is heavy and poor.

[0126] In the golf ball of Comparative Example 5, since the coverhardness is low, the deformation amount is large, and the reboundcharacteristics are degraded, which reduces the flight distance. Inaddition, the shot feel is heavy and poor.

What is claimed is:
 1. A multi-piece solid golf ball comprising a corecomposed of a center and at least one intermediate layer formed on thecenter, and a cover covering the core, wherein assuming that a centralpoint hardness of the center, a surface hardness of the center, ahardness of the intermediate layer and a hardness of the cover in ShoreD hardness are represented by H_(M), H_(S), H_(I) and H_(C),respectively, the H_(M), H_(S), H_(I) and H_(C) satisfy a correlationrepresented by the following formula: H_(M)<H_(S)<H_(I)<H_(C) and thecover is formed from polyurethane material as a main component, and hasa hardness in Shore D hardness of 45 to
 60. 2. The multi-piece solidgolf ball according to claim 1, wherein the golf ball has a deformationamount of 3.0 to 4.0 mm, when applying from an initial load of 98 N to afinal load of 1274 N.
 3. The multi-piece solid golf ball according toclaim 1, wherein the polyurethane material comprises polyurethane-basedthermoplastic elastomer as a main component.